Climate change has become a big issue in recent decades, and one of the major indicators that many people point to as a worrying potential problem is the rise in sea levels globally. There are island nations buying up land in foreign countries, people moving further inland, worse floods every year from tropical storms and hurricanes – yet perhaps an even more worrying problem is that the land itself is SINKING!

“Many of the world’s largest deltas are densely populated and heavily farmed. Yet many of their inhabitants are becoming increasingly vulnerable to flooding and conversions of their land to open ocean. The vulnerability is a result of sediment compaction from the removal of oil, gas and water from the delta’s underlying sediments, the trapping of sediment in reservoirs upstream and floodplain engineering in combination with rising global sea level. Here we present an assessment of 33 deltas chosen to represent the world’s deltas. We find that in the past decade, 85% of the deltas experienced severe flooding, resulting in the temporary submergence of 260,000 km2. We conservatively estimate that the delta surface area vulnerable to flooding could increase by 50% under the current projected values for sea-level rise in the twenty-first century. This figure could increase if the capture of sediment upstream persists and continues to prevent the growth and buffering of the deltas.”

Chao Phraya, (see image above) the river which flows through Bangkok is one of the worst affected – parts of the delta have sunk 15cm (six inches)! Compare this to the global rate of sea level rise due to climate change at only 1.8-3.0mm per year – nearly a tenfold difference!

Scientists estimate that the area of land vulnerable to flooding will increase by about 50% in the next 40 years due to a combination of climate change causing sea levels to rise and land sinking due to human causes.

“This study shows there are a host of human-induced factors that already cause deltas to sink much more rapidly than could be explained by sea level alone.” Journal Geoscience Article

The researchers report that the flow of sediment down to the Chao Phraya delta has been almost entirely blocked, due to irrigation, damming the river, and directing the main flow through just a few channels. In rivers with no dams or man-made controls, the sediment would pass down the river and add to the height of the land, a process known as aggradation. (see image below) Now, the sediment can’t reach many delta areas. The further extraction of water and gas for irrigation, drinking, and industry further compacts the land.

As reported in the BBC yesterday, “Rivers affected include the Colorado, Nile, Pearl, Rhone and Yangtze. Of the 33 major deltas studied, 24 were found to be sinking. Abouthalf a billion people live in these regions…”

THE HIGH-RISK LIST

Deltas with “virtually no aggradation (supply of sediment) and/or very high accelerated compaction”

Chao Phraya, Thailand

Colorado, Mexico

Krishna, India

Nile, Egypt

Pearl, China

Po, Italy

Rhone, France

Sao Francisco, Brazil

Tone, Japan

Yangtze, China

Yellow, China

As the ground falls and sea level rises, people become more vulnerable to inundation during storms.
“Every year, about 10 million people are being affected by storm surges,” said Irina Overeem, another of the study team from the University of Colorado.

So should we be worrying about the inevitable rise in sea levels? Or more focused on the major impacts we are still having on these sinking river deltas, which around the world are home to almost half a billion human beings?